This invention relates to a brake system for an electric motor-powered vehicle which uses both regenerative braking and hydraulic braking, and more specifically a brake system in which brake operating feeling has been improved.
In electric cars and hybrid cars which also use an engine, regenerative braking is adopted in which the electric motor for driving wheels functions also as a generator during braking to generate braking force while collecting energy. But since the braking force obtained by regenerative braking is limited, hydraulic braking is also used.
With a brake system using both regenerative braking and hydraulic braking, it is proposed to maintain good brake feeling even during changeover from regenerative braking to hydraulic braking in Japanese patent publication 11-198786.
In the brake system of this publication, in order to eliminate solid or rigid pedal feeling as felt in the system of Japanese patent publication 7-336806, and sudden extension of the pedal stroke during changeover from regenerative braking to hydraulic braking as felt in the system of Japanese patent publication 5-176407, between the master cylinder and the wheel cylinders, a hydraulic pressure limit/changover means is provided in which a differential pressure valve, an on-off valve and a proportional pressure-reducing valve (or proportioning valve) are arranged in parallel, and on the downstream side (wheel cylinder side) of the hydraulic pressure limit/changeover means, a reservoir for pressure increase is provided to suck up brake fluid in the reservoir and supply it to the wheel cylinders when a pressure-increase command is issued from an electronic control unit.
In the brake system of Japanese patent publication 11-198786, the reservoir for pressure increase is used as a stroke simulator to eliminate solid or rigid pedal feeling. Also, it is possible to cancel a predetermined differential pressure produced between the master cylinder pressure (hydraulic pressure produced in the master cylinder) and the wheel cylinder pressure (hydraulic pressure in the wheel cylinders) during regenerative braking. Thus it is possible to also eliminate sudden extension of the brake pedal (which is felt when the master cylinder and the wheel cylinders are brought into communication with each other to change over from braking using both regenerative braking and hydraulic pressure to braking by hydraulic pressure only).
But since the reservoir for pressure increase is provided on the downstream side of the hydraulic pressure limit/changeover means as viewed from the master cylinder, it is necessary to keep the split point hydraulic pressure of the proportional pressure-reducing valve in the hydraulic pressure limit/changeover means above the actuation-start pressure of the reservoir for pressure increase, and increase the pressure-reducing ratio of the proportional pressure-reducing valve to take in fluid into the reservoir for pressure increase. Thus start of regenerative braking is delayed, and the braking force by hydraulic pressure in the initial period of start of regenerative braking tends to be larger than required.
Also, during braking, since inflow of fluid into the reservoir for pressure increase especially during sharp braking is limited by the upstream proportional pressure-reducing valve, solid pedal feeling is temporarily felt.
Further, when the brake pedal is returned during regenerative braking, brake fluid in the reservoir for pressure increase flows out through a check valve on the pump suction side, the pump and a check valve on the pump discharge side. Thus fluid may remain in the reservoir for pressure increase. In such a case, the brake pedal is not smoothly returned.
An object of this invention is to eliminate such disadvantages and improve energy saving and brake operating feeling.
According to this invention, there is provided a brake system for an electric vehicle which uses both regenerative braking in which braking force is applied to wheels by an electric motor for driving wheels, and hydraulic braking in which braking force is applied to the wheels by supplying hydraulic pressure of a master cylinder to wheel cylinders, and during the regenerative braking, a predetermined differential pressure is produced between the master cylinder and the wheel cylinders by lowering hydraulic pressure supplied from the master cylinder to the wheel cylinders by means of a hydraulic pressure limit/changeover means comprising a differential pressure valve, an on-off valve and a proportional pressure-reducing valve provided parallel to the differential pressure valve, and at the end of regenerative braking, the differential pressure being cancelled by sucking brake fluid from a reservoir for pressure increase, which serves also as a stroke simulator, by means of an electric pump and supplying it to the wheel cylinders in response to a pressure-increase command from an electronic control unit, characterized in that the reservoir for pressure increase communicates with a main flow passage at a point upstream of the hydraulic pressure limit/changeover means, and an on-off valve and a check valve for allowing only a fluid flow from the reservoir for pressure increase toward the master cylinder are provided parallel to each other in a flow passage leading from the main flow passage to the reservoir for pressure increase.
In this brake system, a module for antilock control may be provided for pressure increase and reduction control of the wheel cylinder pressure in response to a command from an electronic control unit provided between the hydraulic pressure limit/changeover means and the wheel cylinders. As the module for antilock control, it is preferable to use a mass-produced antilock control unit comprising a reservoir for pressure reduction for introducing fluid discharged from the wheel cylinders, and an electric pump for sucking brake fluid in the reservoir for pressure reduction and returning it into the main flow passage.
Also preferably, a differential pressure generating means comprising a differential pressure valve and an on-off valve arranged parallel to each other is provided in the main flow passage on its wheel cylinder side of the hydraulic pressure limit/changeover means, and the on-off valve of the differential pressure generating means is opened and closed to change over the differential pressure produced between the master cylinder and the wheel cylinders at least in two stages.
Further preferably, a flow passage for communicating a back-pressure chamber of the reservoir for pressure increase with a reservoir of the master cylinder and a pressure-responsive valve for opening and closing the flow passage are provided, the pressure-responsive valve is acted by hydraulic pressure in the back-pressure chamber of the reservoir for pressure increase in the valve-opening direction and by the hydraulic pressure on the wheel cylinder side in the valve-closing direction so as to open when the differential pressure produced between the master cylinder and the wheel cylinders is below a predetermined value, and close when the predetermined value is exceeded.
Since the reservoir for pressure increase is provided upstream (on the master cylinder side) of the hydraulic pressure limit/changeover means, the proportional pressure-reducing valve included in the hydraulic pressure limit/changeover means can be provided only for the purpose of replenishing brake fluid equivalent to fluid returned by piston back/retraction of the wheel cylinders. It may be one in which the split point hydraulic pressure is low and the pressure-reducing ratio is small or zero, and it is possible to make the hydraulic pressure at the start of regenerative braking unlimitedly close to zero.
Also, since the reservoir for pressure increase, which is also used as a stroke simulator, is provided upstream of the proportional pressure-reducing valve, so that brake fluid from the master cylinder flows directly into this reservoir, solid pedal feeling will not be felt even during sharp braking.
Further, if the brake pedal is returned during regenerative braking, since brake fluid in the reservoir for pressure increase returns through the check valve and to the master cylinder, remaining of fluid in the reservoir for pressure increase will not occur, so that no hitched feeling of the brake pedal will develop.
With the system using a mass-produced antilock control unit as the module for antilock control, although it is provided with two electric motor, using a mass-produced antilock control unit is inexpensive than using a single electric motor both for pressure increase during regenerative braking and pressure re-increase for antilock.
Also, with a system in which the differential pressure generating means is provided, it is possible to change braking force by hydraulic pressure in a stepwise manner. Braking force obtained by regenerative braking is, due to the characteristics of the motor, large in a low-speed region, and gradually decreases as the vehicle speed rises as shown in FIG. 7. Also, it decreases sharply from a certain point of time in which the vehicle speed approaches zero. Thus, it is preferable that between the braking from high speed and braking from low speed, the braking force by hydraulic pressure differs. With a system provided with a differential pressure generating means, it is possible to control so that the braking force by hydraulic pressure is large during braking from high speed and is small during braking from low speed. This permits braking close to ideal.
Also, if adjustment of the amount of brake fluid taken into the reservoir for pressure increase is carried out by controlling the on-off valve on the inlet side, a pressure sensor for detecting hydraulic pressure is needed. In contrast, with a system in which a circuit for bringing the back-pressure chamber of the reservoir for pressure increase into communication with the reservoir of the master cylinder is provided and a pressure-responsive valve is provided in this circuit, when a predetermined differential pressure is produced between the master cylinder and the wheel cylinders, the pressure-responsive valve closes and taking-in of fluid into the reservoir for pressure increase stops automatically. Thus, no pressure sensor is needed and control of the system is simplified.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: